Switch to custom fcrepo container image

This uses:
- the latest release 4.7.6 from fcrepo's 4.7 version branch
- the latest Tomcat 9 release that still supports Java 8

The previous image "ghcr.io/samvera/fcrepo4:4.7.5" has never actually
worked, so this new image restores compatibility with the original
"ualbertalib/docker-fcrepo4:4.7" image.

Also remove unneeded variables from .env.template and
.env.template.development.

Fixes: a6a58dcd "Upgrade fedora image in docker"
Fixes: #19
Fixes: https://gitlab.ruhr-uni-bochum.de/FDM/rdm-system/antleaf-projectmanagement/-/issues/216